Electromechanical-conversion device



Feb. 20,1945.- y H T 12,369,676

ELECTROMECHANICAL- CONVERS ION DEVICE Filed Jul-y 10 1940 4 Sheets-Sheet1 Feb. 20,1945; F, v HUNT'ETAL 2,369,676

v ELEECTROMEGHANICAL-CONVERS ION DEVICE Filed Ju-ly'lO, 1940' 4Sheets-Sheet 2 J'aH/v ALw/v P/ERcE Feb. 20, 1945.

F. v. HUNT ETAL 2,369,676

ELECTROMECHANICAL-CONVERSION DEVICE Filed July 10, 1940 4 Sheets-Sheet 3f'msasmcw l/. Hu/vr JbH/v ALVIN PIE/7C6 Patented Feb. 20, 1945 aseasrsFrederick Vinton Hunt, Belmont, and John Alvin Pierce, Cambridge, Mass.

ELECTROMECHANIC AL-CON VERSION VICE Application July 10, 1940, SerialNo, 344,750

7 35 Claims.

The present invention relates to electromechanical-conversion devices,and more particularly to vibration-translating devices, such,

for example, as are used for reproducing or recording sound with wax,shellac, lacquer, metal, and similar phonograph-record materials.

In Letters Patent 2,239,717, granted April 29,

1941, there is disclosed a new and improved phonograph reproducerthatreproduces phonograph records with complete fidelity, encompassingthe entire range of audible frequencies, with very low distortion, andwith very small wear or the material of which the records areconstituted.

An object of the present invention isto im prove upon reproducers of thetype described in the said Letters Patent. v

A further object of the present invention is to provide a new andimproved phonograph reproducer that shall be easy to fabricate and thatshall be sufficiently simple in mechanical behaviour to allow aquantitative mathematical analysis of the behaviour and a quantitativeengineering design in advance of actual construction.

The reproducer disclosed in the said Letters Patent employs viscousdamping material that, however, may become subject to aging anddeterioration. A- further object of the present invention, therefore, isto provide a phonograph reproducer which shall require no viscousdamping material.

Another object is to provide an electromechanical conversion deviceembodying a novel jack, or clamp, whereby the action of a screw inpressing together two pairs of conductors also provides astron'g'mechanical clamp, thereby relieving the conductors themselves ofmechanical strain.

Other and further objects will be explained hereinafter and will beparticularly pointed out in the appended claims.

The invention will now be described in connection with the accompanyingdrawings, in which Fig. 1 is a plan of a phonograph reproducer embodyingtheinvention, shown disposed in contact with the surface of a phonographrecord; Fig. 2 is a section, upon a larger scale, taken upon theline 22of Fig. 1, looking in the direction of the arrows; Figs. 3 and 4 aresimilarly enlarged sections, taken upon the lines 3 -3 and 4-4,respectively, of Fig. 1, looking in the direction of the arrows; Fig. 5is a fragmentary I described hereinafter.

of Fig, '1; Fig. '12 is a plan, similar to the plan of Fig. 5,illustrating a preferred embodiment of the invention; Fig. 13 is adeveloped plan of the unitary ribbon inductor and mechanical supportinmember used in the embodiments of the invention illustrated in Figs. 12to 18, inclusive;

Fig. 14 is a section of the reproducer head, taken upon the line l4-'-l4of Fig. 12, looking in the directions of the arrows; Fig. 15 is asection of the jack for-mounting the reproducer head of .Figs. 12 and 14on the tone arm, the section being taken upon the line l5l5 of Fig. 14;Fig. 16 is an underside plan of the reproducer head illustrated in Fig.14; Figs. 17 and 18 are sections of the reproducer head, taken upon thelines I'l-l'l of Fig. 16 and lB-l8 of Fig. 14, respectively, looking inthe directions of the respective arrows; Fig. 19 is a schematic planrepresenting the structures shown in detail in previous views; Fig. 20is a correspondin schematic elevation; Fig, 21 is an electric-circuitdiagram equivalent to the mechanical system of Figs. 19 and 20; andFigs. 22, 23, 24 and 25 are symbolic graphs representing performancecharacteristics more fully A stylus needle 3, the lower'end of which isadapted to be disposed in contact with the undulations in the soundtrackof a phonograph record I, is mounted at the front end of a movable tonearm. As explained in the said Letters prise the tone arm and arepivoted,as described in the said Letters Patent, to permit the tone arm to swingfreely back and forth, and also up and down.

' As illustrated in Figs. 1 to 4, the front open ends "of the tubes 5may be held together by a yoke '35, carried by an insulating bottomplate 18. The yoke 35 and plate l8'may be constituted of Bakelite. Theplate 18 may support a highintensity of U-shaped permanent magnet 54havplan of a modification, the magnet and otherv parts being omitted,for clearnes's; Fig. 6 is a corresponding elevation, partly in section,upon the line 6-6 of Fig. 5, looking in thedirection of the arrows; Fig.'Lis an underside plan of the forward portion of the embodimentillustrated in Figs. 5' and 6-; Figs. 8, 9, 10 and 11 are sections takenupon the similarly numbered'section lines ing a north-pole piece 68,providing a north pole N, and a south-pole piece 10, providing a southpole S. A small stationary soft-iron pole piece 11 is held in place onthe insulating plate 18 by a pin or screw 9. A permanent horse-shoe mag-. net 1 ill of somewhat different shape, having softiron pole piecesH2, and a centrally disposed pole piece H4, is shown in Figs. 12 to 18.

between the pole pieces N2 of Fig. 18. It may be constituted of apivotless-inductor dynamic-type device in the form of a substantiallyhorizontally disposed single-turn or multiple-turn inductor, shown asthe elongated loop l3 of Figs. 1 and 2 and at 38 in Figs. 5, 6, 16 and18. The loop constitutes a combined electric circuit and mechanicalsupport. The sides of the loop are shown disposed transversely, in themagnetic field of the permanent magnet 54 or I I0.

The inductor loop 38 shown in Figs. to 18 is in the form of aninterrupted cylindrical shell, provided with rear-end supporting arms66, one at each end of the interrupted cylindrical shell.

In the modification shown in Figs. 1 to 4, the rear ends 66 of the loo-pI3 are secured to mounting blocks 80 that are held to the insulatingbottom plate 18 by screws 64, and they may be cone nected to atransformer (not shown) by wires 58 and 60 that may be insulated fromeach other in any well known way. The inductor element is provided withan intermediately widened portion, bent into the form of a stiff conicalshell 36 at the extreme forward or outer closed free end of the loop l3.Ribbon-like arms 82 and 84, of any desired length, connectthe shell 36to the ends 66 of the loop. The active portion of the loop thus extendsthroughout the length of the pole pieces 68 and 10. a

As is further explained in the said Letters Patent and embodied in thedevices here disclosed, the stylus 3 is rigidly mounted at the bottomapex tip of the cone 36, substantially normal to the substantiallyhorizontal plane of the inductor loop, and the upper base of the cone issubstantially in the plane of the inductor loop. The height .of the coneprovides the necessary leverage, and the cone shape provides asubstantially maximum-stiffness lever for the mass of material employedfor the purpose, inasmuch as a conical shell of given mass provides astructure of the maximum stiffness to connect the stylus point and aloop of given size. The conical shell 36 thus connects the stylus-3 andthe loop driven thereby, to provide a very high ratio of stiffness toinertia, leading to small phase shift between the impressed motion andthe electrical output, and correspondingly excellent transient response.

Whether constituted of a single loop as here illustrated or, wheregreater voltages are needed, a plurality of loops as described in thesaid Letters Patent, the inductor coil is easy to fabricate, since itmay be made from a thin ribbon blank, as shown in Fig. 13, constitutedof some electrically conducting metal, such as duraluminum or,preferably, coin silver, that may be punched from sheet stock. When bentinto shape, the ribbon is substantially vertically disposed to providegreat stiffness and rigidity.

In the modification of Figs. 1 to 4, the rear arms 66 of the loop aretwisted out of the vertical plane toward the horizontalby (90-A)degrees, as illustrated in the sectional view of Fig. 3. As described inthe said Letters Patent,but not illustrated here, the side arms 82 and84 of the loop I3 and the upper or base portion of the conical shell 35may be bent into the form of a U-shaped channel. The function of thischannel form is to provide great rigidity of theforward portion of theinductor loop. In the modifications illustrated in Figs. 5 to 18, evengreater rigidity is provided by securing the interrupted cylindricalinductor portion 38 of the loop about the cylindrical upper base portion31, which is integral with and partakes of the rigidity of the conicalshell stylus support 36. This rigidity provides that the forwardportion, or the inductor loop, which lies in the magnetic field, shallalways move as a single whole, "without relative vibration of theelectromagnetically active portions of the loop. On account of thedisposition of the loop ends 66 in substantially the same plane as theremainder of the inductor loop, the active portion of the loop is verystiff for lateral forces applied in the plane of the loop. However,because the loop ends 66 have the form of ribbons whose width is largein comparison with their thickness, the inductor loop has greatflexibility for motion occurring about a longitudinally disposed axis ofthe loop. When a lateral force or displacement is applied to the stylus3 positioned at the apex of the comcal shell 36, the only motion whichthe electromagnetically active portion of the loop can execute is arotation about the longitudinally disposed axis, and because theinductor-loop structure has great flexibility for thistorsional motion,the force required on the stylus 3 to produce this motion of the loop isextremely small.

The mass of the inductor-loop element, together with the conical shelland the sapphire stylus, may be made less than approximately 30milligrams. Most of this mass, owing to the shape and disposition of theinductor loop, is concentrated close to the longitudinally disposed axisof vibration, so that the equivalent mass of the en-. tire vibratorysystem, as viewed from the stylus apex as a driving point, may be lessthan about 5 milligrams. The mass reactance of the dynamical movingsystem, even at the highest frequencies, is thus so small that verysmall forces are required to hold the stylus positive contact with therecord groove. In consequence of the small mass 'reactance of thevibratory system, the wearing of even very soft record materials isminimized and the transient momentary deformation of the record materialis so slight that very high frequencies are reproduced withoutappreciable translation loss. v

A very light, unrestricted vibration-translating device is thusprovided, adapted to vibrate about an axis which is quite rigid andstifi for lateral displacement in the plane of the loop, but which isvery flexible, so as to move substantially freely, in torsion; a lateralvibrational displacement of the stylus 3 below the plane of the loop isconverted into a rotary motion of the stiffened front portion of theloop. No pivots are required to obtain the desired motion of theinductor loop since the longitudinal axis of vibration is established bythe inherent differential fiexibilities obtained by means of theconformation of the mechanicalsupporting-and-inductor loop itself. Asthe stylus 3 moves up and down, and to the right and the left, thestylus yields and,moves freely in all directions, communicating theappropriate vertical or rotational motion to the inductor loop by virtueof the inherent flexibilities obtained by the conformation of the loop.

The loop I3 or 38 constitutes both the mechanical system to which thestylus 3 delivers its energy and the electric system for conveying theenergy. A voltage is induced in the loop when the stylus needle 3 iscaused to vibrate to drive the stylus support and, by conventionaltransformation, the electrical energy may be delivered from thelowimpedance loop to ordinary Communication circuits. The conical stylusmount and the inductor loop are so light and have such great rigidity onaccount of their geometrical configuration that the electromagneticallyactive system vibrates I ascacrze' dition to providing therequireddampingxof the free. vibrations of, the inductor loopinduced-(by freely as; a "rigid .unit. response .to the force exerted onthe stylusbythe'soundttrack. 3:1:

" ,Sudden-Ijars. of the turntable, ortone-arm mounting might be thoughtby some .to be liable to cause the reproducer to bounce out of therecord groove.v As is explained andclaimed inzthe said Letters Patent;the .mass distribution and pivoting of.the'tone arm maybe arrangedin.suchaway as to avoid this difllculty. As is further explained-in thesaid Letters Patent, the stylus-supportingarm may be made suf-;ficiently. long so as to enable. it to swingsomewhat beyond the centerofthe record, thus reducing the error introduced by change. in thetrack-.v

ingangle: Theloopl3 is shown disposed at,;an

angle tothe direction of extension of the suppo-rting arm;to; reduce thetrackingsangle to: amini-;

mum, or low, value.

1 the stylusmotion, provide'a means for transmite ting a;portion'of theforce pressingqth'e stylus 3 into {contact with the record groove; thesaid force, 1 being produced by i the i net unbalanced weight of thereproducerhead and tone-arm. Also described as a modification in thesaid Let-J tersPatent, there was provided an. auxiliary spring (notshown here) for transmittingthis force pressing the stylus into contactwith the record groove. It is undesirable torely uponthe L'As explainedin the said Letters Patent, -.ane

described more fully in. our published, paper in .the Journal of theAcoustical Societygof America for July, 1938 (vol. 10., page 14) it isnecessary for a phonograph reproducer stylus to be fiexiblein thevertical direction in order to;avoid deforming therecord-material in thepinched sections ofthe groove. According to the, present invention, thepressure exerted by the stylus needle .3fon the.

reoord-andthe mechanical impedanceof the dy,

namical system for vertical 'motion are so small that, ,instead oftearing through the material of the record in the pinched sections ofthe groove, the stylus rises up slightly andpassesoverthemwithoutcau'sing any damage; and-this without failing to followsubstantiallythe centerline. of the recordgroove. This results inthetotal elimination of all even harmonics in the lateral motionposition of the magnetic field, as illustrated, the otherwisedistortion-producingvoltageisthus inmechanical strength of the saiddamping'mate I rial to transmitthis force on account of thepossibilityof cold-flow of the damping material, with consequentderangement ofthe, relative positions of the-reproducer parts. I

According tothe present invention, ,,therear portionsof the vibratoryloop are inthe form of two flat ribbon members 66, each ,inclinedto-thehorizontal by an angle A, as indicated'inthe sectional view of Fig. 3,toform between them a dihedral angle. ('180.- -2A). A sufficient elastictrengthin the vertical direction may thus be secured -to carry the saidunbalanced ,weight. Atthejsame time, and by virtueof the ribbonlike formof the members 66,-,there is'no appreci able stiffening, of thestructurefor rotationalmotion. By, virtue ofthe inclination of the meme;

vb'ersv 66 to the horizontal by theindicated angle, the same desirablefeatures of rigidity forlateral displacement in the plane of thevibratory loop; and great flexibility in torsion about an axis lineparallel tothis plane, are secured, with the addi-- tional provision ofasuitable vertical-stiifness.

I This vertical stiffness can be altered by altering the said dihedralangle, and rotation takes place about a horizontal axis lying at theintersection of the two planes of the inclinedribbon members 66, Theunbalanced weight of the reproducer head may .thus be supported by thevertical stiffnessdevelopedby this inclination. The damp ing materialhas no function,to *perform-except auxiliary springs to transmit averticalcompo;

troduced into the twosides of the loop inopposite directions, andissimultaneously accurately cancelled out, .so that onlythe voltagecaused by thev lateral motion of the stylus needle. remains to producethe desired electrical output, correspond-' ing in facsimile to therecorded groove modula: tion... I .1 ;.According to the inventiondisclosedin the said Letters Patent and embodied in the modificationillustrated, injFigs. l dto l, damping; is provided; partlyby a dampingmembrane [9 connecting the cone 36 at theflclosed, outer end'of the.looptothe pole piece 1'! and lying within the loop 5|3, and

' partly bytwo similar membranes 2l and123.-The

membrane l9v is disposedat the extreme forward to providedamping andthereis, therefore, no difiiculty arising from cold-flow of the dampingmaterial, nor any necessity to provide additional nent of force; a a vIn Figs. 5 to 11, the conical shell 36 is shown as separate from itsinductor-andelastic supporting ribbon, and-as embodying the upper cy- 1lindrical section 31. It is, however, combined into a unitary. vibratorystructure with its supportend of the loop, at the points where. thecone36 is joined'thereto, almost on the axisofrotation, where the amplitudeof motion is least,. and anyefiect cinch-uniform elasticity of thematerial is minimized The membranes 2| and Bare disposed at theextremeen'ds of the loonand connect-the ends 66 ofthe loop I3 withthe-rear poritions ofthe adjacent magnetic structure or the base 18,bridgingthe spaces'between them; .lThe. membranes ,2l, and 23 absorb'andprevent-refiece tion ofs'uch dampedivibrationsas may reach td-thesepoints; ,1--; .v dam n membranes, '2! an ih ing-ribbon by wrapping anintermediate narrow portion 38 .of the latter around the major portionof the, periphery of the cylindrical section. The supporting ribbon is.bent' at its junction lines 45 with the cylindrical shell 31 to formthe two substantially flat supporting and conducting members 66,hereinafter calledthe'Fbackeflats. The cylindricalshell 3'!v iscustomarily insulated from the inductor-.38 by the cement used t'ojointhem into. a unitary structure, but this insulation is not important. atthis junction because the inductor 38 may be of high electricalconductivitywhile the cylindrical shell 31 is very thin and oflowconductivity, and very little loss of output is brought about byimperfect insulation between them. The developed form of the unitaryinductor and-mechanical supporting member may be similarv to that shownin Fig. 13. The preferred embodiment, illustrated in Figs. 12, 14and-16, isthe same in essence. as thatillustrated by'Figs. 5, ,6

and 7, except that the back flats have been -ta-" p'e're'd to secureadded stiffness-ofthe transverse compliance on therequivalent pivot. Amagnetic field: late 1 be provided in the structures of Figs; fto'11;the sameas illustrated in Figspl2Lto 18,

and-hereinafter:described. Of the unitary in-l ductorandmechanical-supporting member, the narrow interruptede'cylindrical-shel1portion 1 I 38,

wrapped aroundthe cylindricalshell 31 consti i tutes the inductorportion;- while i the "widened supporting-arm portions 56, extendingbeyond thetwo junction lines 45,'one=at each end of theinterrupted-cylindrical shell, constitute the. elastic-conductingandesupporting member.

in Fig. 13,- extending beyond lines 43, provide portions-forsclamping,welding, or soldering to fixed conducting supporting elements 65.

The conductors 58 and 60may be respectively connected to the supportingelements 65. The conducting leads '58 'andaMTmay be held to aninsulating supporting block- 1| by a screw "13.

A supporting-tube'1 5 is held on the tone arm'by a screw 11, ina matingtube-83 The tube 15 carries the insulating supporting "block- 1|. The

Tabs 1 "j shownrectangular in- FigL' but triangular tubes and'83 may beclamped together by the screw'11." The tube 15,the supporting block1|,and the vibrator-y I elements and the mounting supports carried thereby,may be removed from the tonearm byunscrewing the screw'11i The tube 15contains theleads 'SBand fifljinsulatedby the. supporting block 1|,andthetube- 83'two corresponding spaced mating leads 85' and '81,"insula'tedtrom the tube 83. The conductors 58,

80, 85" and 81 are-thus disposedhetween' the clamping members 15 and 88.The screw 11 extends into *the'tone armthrough a-slot 19*"in the tube15'.-

The-end of thetone arm on which thereproducer head ismounted comprisesan outer respectivemating'lead conductors and-6O from the reproducerhead; at the same time, the force pressing-these leads together acts tospread the split: inner tube 83 into close mechanical contact= withtheouter: shell. 15 of the reproducer head; The singleoperation oftightening the screw .11 i affords a good xelectri'cah contact betweenthe leads and a strong-mechanical attach ment 'of. the-reproducer'headon the' tone arm without reliance on" the electrical: conductorstormechanicalstrengthn The operation 1 ot. .the repro ducer: may be"better understood'in connectionwith the schematic rep": resentationsof:Figs. 19 andzzo, wherethe rela tively short and'wide back-flatsupporting mambe'rsiG are" presented as disp'osed-instwo planesintersecting along. a line'perpendicularrtotthe planeof Fig; 20',-asindicated at. P; Theback flatmembersfifi, as viewed'in the horizontalplane of Fig. 19, may be disposed approximately at right angles to each:other. This 90 angleissapproxi mately. the optimum angle. for providing.maximum transverse stiffness rot the eflective :pivot' with a: low.stiiin'ess for torsional rotation of .the. inductor.

A-sstructure is. thusprcvided thatislvery rigid? ammo 101:. .lateraldisplacement-int the plane of Fig..19, but veryl-iiexible'in torsionabout the said axial line'through Pi ins.=Fig. 20. As heretofore described inconnection: with Fig. 3, so'also in Fig. 9 andrEiga20g: theangle/A of inclination of the back-ifla'timembersxfio to the .horizontalwill providesthe necessary verticalhstiifness to. transmit the'forceipressingzthe istylus 3 into contact with the record groove.

In connectionawith the featuresof the present invention'ithat relate toprovision f or positive tracking," we make particular-reference to Fig.2 ot our saidpaper'in theJournalof. the Acoustical Societyiof: America.Ior July, l938 (vol. 10, p. 14). There it is. shown that the includedangle of a record groove which is laterally'modulated must.

change as the recording. stylus. executes .a motion perpendicular to therecord groove. 1 In order forthe motion of the stylus 3 to be entirelycontrolled .by the modulationof the record groove,

it-will be cbvi'ous that at least a portion of the recordigroove oneachside ofthe midline of the stylus '3 must be in continuous contact withthe stylusat all ti'm'es. Such'a condition of positive guidance-of thestylus is sometimes called positive tracking and it will be obvious thatwhen this condition is satisfied the stylus needle 3, in executing-thelateral motion corresponding to the record groove. must'also execute avertical motion ass, result of the change in the included angle of therecord groove.

The structures disclosedherein make provision for this vertical motionand the magnetic field, represented'symbolically by the arrow H+H inFig. 19, is symmetrically disposed, so that no net voltage is induced inthe inductor by vertical motion'of the stylus In order to hold'thestylus in positive contact with the record groove at all times, and forany modulation impressed upon the-record groove; it is necessary thatthe vertical" compliance afforded by the elastic supportingzmembers 86fall'within certain limits. These limits'are'determined by theconsideration that the force necessary to deflect the stylus verticallyfrom itsequilib'rium position by about 0.01 inch should be capable ofimparting to the efiective mass of the vibratory element for verticalmotion an accelerationvof about five hundred times that due togravity;In a-similar way, the desirable value of compliance for transversemotion of the stylus: must be-so 'chosen that the maximum displacementfrom' equilibrium produced by lowfrequency modulations of the recordgroove will not require transverse forces greater than that pressingthestylusin'contact with the said record groove. indicates that thetransverse mechanical complianceLat.the styluslshould lie between oneand two times the mechanical compliance at the stylus forverticalmotion; i

' Thewdimensional proportions and the disposition. ot-ithe-.:back-'flatmembers 86 necessary to I secure :thisidesirable.proportion of transverse and For typical applications, the foregoingaseaeve v f "the supD rting block .1 (Fig. 6)}: -.'The face: 9 3 of thesupportingblocki'll imaybe butisnot'nec essarily in aplanerperpendicularztothe axis of the cylinder'31." The back-flatsfitindicated in Fig; 19 may be twisted out or a 'plan'e through the useof an angle on -(Fig.8 )i-att.the point of attachment of the back-flatsto the cylindrical section, different from the angleez (Figkll) v formedat the rear of'the back-flats, where the back-fiat rii'ember isattachedi to the mounting blocks.

In actual practiceit' is notnecessary that the axis of vibration bestrictly parallel to the. surface of the recordl. In the mathematicaltreatment based on Figs, 19 and '20, however, such l parallelism hasbeen assumed for convenience. An important advantage is gained byinclining the axis of vibration vvith respect to the record surfaceinasmuch as such' inclination maybring the point'of attachment of theback-flats 66 closer to the recordsurface. This has the advantage ofproviding that when, thestylus. is defiecte'd upward thecompon'en't'ofthe resulting relative-motion along the record groove isj'reduced.Moreover, there 'is-inevitably some fric- -tional drag induced by thepassageof the record groove under the stylus tending to deform theelastic support for the conical shell. vIf this deformation is'presentto any considerable extent, a to-and-fromotion of the stylusalong thedirection of motion of the record groove may be excited in a mannersimilar to that in which a 7 from entering theprotective-housing;:vvhere it might enter the narrow'air gap in the magneticcircuit andinterfere with the satisfactory operation of the reproducer;We have found the following described disposition of the back-flats toprovide suflicient stiffness for the longitudinal.

--motion of the stylus tdm'ake these auxiliary-provisions unnecessaryinmost cases. However,for

' reproducers or'vibration-translating devices constructed according tothe present invention but with dimensions-different from those-we haveused in our experimentalwork, or in devices designed for otherfrequency'ranges, these auxiliary restraints maybe desirablefand ourexperiments and our analysis 'have'in'dicated the adequacy of thementioned methods for avoiding'such diflic'ulties. i v

violin string is setiinto vibration by the gliding bow.

I This to-and-fro motion of the stylus apex in;

and along the direction of, the record groove and z the resultingmotion'of the inductor portion of the vibratory elementproduce noelectrical output by virtue of theisymmetrical disposition of themagnetic 'field. .HoWever, this motion has the effect of superimposing afluctuation upon the uniformlinear velocityof the recordjgroove andimpressing upon the electrical output of the reproducer aflutter orfrequency ;modulation. 5

- In the present invention, .We disclose anddescribe" amethodof-disposingthe back-flat members 66 through choice, of th anglesn and a2, aswillbe described below, and of; shaping these members sothat a very low .mechanical compliancefor I longitudinal motion ofthestylusis secured. It

is possible to restrain thislongitudinalmotion of the stylus along the,record groove in several'other' Ways such as, foreXample'byprovidinga-flexible wire or ribbon member. We have found that the longitudinalmotion of the stylus'may be damped effectively by using a restrainingmember com- The term back-flat, throughout this description, hasbeen-used to referto theelastic-supi porting-and 'conductingmembersBG-{Whether or *notthese are in a single plane. In thesi'mplified case,inwhich "11. p r

the. banana has is 1181mm time. .the I mounting face 93. Undertheseconditions, arotation of the conical. shell 36 about the axis throughthe. line P' (see 20.). formed by the intersection of the )planes of theback-flats (and,

therefore, inthe plane of the back-flats, since they are in this casehorizontal) involves bending ofthe back-flat'members 66-ofthefixed-fixed type. .By fixed-fixed type is. meant that. form ofelastic deformation oftheribbomhke beams,

in which the vend portions are constrained to re- ,main approximatelyparallelv to each other.q ;On

the other hand, fora vertical displacement ,ofthe stylus apex 3, thebending. of the back-fiats fifi is of the fixed-free type,-' as inan-ordinary'canti- 1 lever beam loaded at one end. If the; effective:len'gths'of theback-flats are-the same; for these two types .ofdeformation, @the compliance Ct v for vertical motion .would beapproximately eight times the compliance Ct for transversemotion of thetylus. The above-described considerations, however,: have. indicatedthat this would provide anundesirable relation between the verti cal andtransverse compliances, and that itis desirable in a phonographreproducer, that the compliancefor transverse motion of the stylus tipshould'be between one ,andstwoi timesithat for vertical motion of thestylus tip.

Now, if the angle at the front or theback- I fiat members 66 ismade'difierent from-zero, and

prising a thin filament or'block of a viscous A dampingmaterial22,-such'as1'Visca1o-id, rubber,

Vistinex or other. plastic, attached to theconical shell nearthestylusapex and ,extending'therefrom to the fixed point of attachment20. supported from the insulating block;'l-l;, as illustrated in Figs.6-and 7. Another andpreferred modification of the restraintforthelongitudinal vibration of the stylus may taketheform-of a thinfilm of elastic material '25 surrounding the apex of the conical shellabove its point-of contact with the record groove and. cemented to abut.-

= ton-likev projection 21. on the bottom of the mountingplate ilflshownin Fig; '2, or similarly at-.

tached to the bottom-platecof a protective housing (not illustrated),surrounding*the reproducer head pictured in Figs. 12/60 18. Such anelastic film may also serve the purpose. of preventing dust and smallparticles of extraneous material the vertical and the transversecompliances at the stylus tip are measured and plotted as a function ofthe angle 002 at the rear of the back-flat membars .66, experimentalcurves are obtained similar to' those shown'by the 'solid linesof Fig.22..,,v In

general," the verti'ealfcompliance CV decreases rapidly as the angle on:isincreased from zero toward ninety, degrees. 1.0171 the other hand, the

torsional compliance Ct increases slowly as. a2

' is first increased from Zero, anddisplays abroad maximum. at someangle in approximately; equal to in, this equality occurring-whenthe-back-fiat members 56' become substantial-ly plane. This indicatesthat-fora range of values of az, the

this largerwaluef of :front angie 31x1, the torsional "compliance Ct"again displaysa broad maximum ata-higher valueofmz, againapproximatelyequal -to thew-increased value of. the-angle: ad; n the other. hand; ifv-theyangle-mh chosen too small, fit'may 'be that the maximum of thecurve 0f transverse torsional complianceoccurs at soasmall vava'l-ue of:2 that the verticalcompliance Cv-is greater thanithezcompliance Ct.for. all values of "an, as indicated in Fig. 23. :Itwillbeseen fromtheseflgures that it .is possible to select values of a1. and (:2 suchthat an-y desirable .ratio between,- transverse and vertical compliancemay be secured in the resulting structure. .Havingse- *cureda desirableratio betweenth'ese compliances,

their absolutema-gnitudes may be made to have desired values by anappropriate selectionlof the Width and'wthe thickness of'zthe.bacbfiats-fifi.

ltmaybe pointed out also that an rincreasein :the frontanglewmaatthepoint of attachment of A the' back-fiat i5 tothe conicalshell-provides a desirable stiffening of this junction for the type ofdeformation'described above as a consequence of the longitudinalfrictional force along the record groove. 'If on isincreaseditoo"fartoward 90?; the transverse stiffness" of the efiectivepivot 'would be undesirablyflreduced. ,Our measurements haveindicated"that the angle armaybe chosen intermediate-1y, 'suchas'about so thathigh transverse stiffness 'o f'the effective-pivot .is secured 'while at'the'same" time a high degree of stiffness; or very low complianceCL;isprovided for-longitudinal forces atthestylus apex. The

measurementsindicatedbythe curves arms. 22

"further indicate that the reanangle 122 at. the .t

point or attachment-of the back-flatson their mounting blocksjmay' bechosen "independently to'yield suitable ratios of transverse andverticalcompliance at the stylusapex. "Wehave found a suitable angle :12 to beabout 6, and'a suitable thickness of 'the'ba'ck fiats'to'be about .0015

v inch. It follows that an ofthe elastic complig is the distance fromthe apexlof the-stylus to the center-of gravityof the vibratory system:J; is the'distance fromsthe stylus apex toe line through perpendicularto. the planeof Fig. 20 .and corresponding, if 411:0:2, to the intersecton of the planes containing. the back-flats or, if 1116 112, to the linerepresenting. the axis of rotat on determined by the elasticity of the'backefiats alone m isthe total mass" of the vibratorysystem; Its themoment of inertia or thevibratorysystem about-anaxis perpendicular-tothe'plane of Fig. 20, and-passing through the stylus apex; C is thetransverse compliance in centimeters per dyne or the equivalent-pivotestablished by the differential rigidity of the back-flats 66; C, is the'rotationalzco'mpliance, in radians per dyne-centimeter, providing therestoring torqu for angular displacements. of the'vibratorv about theequivalent pivot;

' finiteaand zero values.

is :the transverse displacement; or the stylus 'apex'ifromits-equilibriumtpositi'onzinduced by .thepdisplacementof the-center'lineof the rec- I ord groove' (y .isnotzillustrated in Fig.20,. be-

cause the stylus is there shown as occupyin its equilibrium position)'inis the equivalent displacement vfrom equilibrium of. the inductorportion transverse to the .mag-

netic'field :-(a: is notillustrated'in Fig. 20 be- -:cause Fig..20represents the equilibrium condition) and grand :1: represent the-timederivatives of y and r, respectively, that is, the velocities o'fthecorresponding displacements. The solution ofv the differential.equations describing the motion of the: system may be representedby'theelectricalcircuitbf Fi .21. In this circuit, 11 asconstantcurrent.generator, repre- "-where o cv I e ghgcla 11, I 1090,

{unis .theangular .frequencyotth impressed transverse displacementy.

.Theperformance zindicated by this expression is represented by'the''symboliccurves of 'Figs. 24 and 25, inwhich the dashed 'lines'atthe'critical frequencies 02' and wrepresent;respectively, in-

In practice, some viscous damping 'is inevitable in a realizablestructure so that these infinite or zero values never occur.

Thevirnportant conclusion tobe drawn from thismathematical analysis isthat the structure may be-so: designed that. the critical frequencies.10: and wo may be :made to'coincide. In such a case,' thecorresponding'terms'involving oz and wo in. Equation 1 canceL'and thefrequency response :ot the reproducer is perfectly uniform up tofrequencies so high that "the assumptions upon which the equivalentelectrical and mechanical systems are based no longer hold. Thisfrequency, in practical realizable structures, lies wellbeyond'theaudible range of frequencies.

'Further analysis of the conditions necessary to make wa m yield *thephysical interpretation that the axis of percussion pf'the vibratorysystem' assa whole should be made to coincide with the elasticallydetermined axis passing through P atthe distance 'hfrom'the stylus apex.Thus, if the line through the-stylus apex perpendicular to Fig. 20 and'the axis of rotation through P defined by'the' elasticity of theback-flats 55 are conjugate centers of oscillation for impulsive forcesapplied either at the stylus apex or at this axisthrough P,1 theresponse of the reproducei' willwbel perfectly uniformw'ith respect tofre- I This desirablev relationship may be quencywithout any necessityfor the provision of damping materials. 1 further explainedby callingattentionto the fact that in .most textbooks on Mechanics (seereferences below) the terms center of percussiorrfland center ofoscillationv are discussed in connection with the motion f"a rigid bodyhaving a fixed and immovable pivoting axis The center of percussion is.then analyzed as that pointqat which the bodymay be struck an impulsiveblow without causing :anyforpe reaction on the pivot transverse. to theline joining the pivot and the point of application of the impulsiveforce. .It is further shown in these textbooks that if the impulsiveforce is applied at the previously immovable pivoting point, then theprevious point of application is an instantaneous center of {oscillationfor the resulting motion; thus the previouspivoting point and theprevious pointxof application are conjugately related and each is aninstantaneouscenter of oscillation for impulses.

applied at the other. 7

Now, in the presentinstance, as in all prior; art vibration-translatingdevices, the pivot is not perfectly immovable but has, for transverseforces, an elastic compliance whose mechanical impedance is notnegligible in comparison .with the mass reactance of the vibratorysystemat the higher frequencies of exitationq. If, therefore, the massdistribution of thevibratory system were such that'the stylus apex orpoint of-application of the impulsive forces were not 'a"cenquency-terms inthe response Equation 1 cancel out as above mentioned.

ter of percussion with respect to the elasticipivot,

then transverse force reactions would occur at the pivot uponjtheapplication of such'impulsive vibromotive forces at the stylus apex."Since the.

vibrator has inertia, these force reactions :at the pivot would "beia-ble-to excite resonance oscillations whosefrequency is determined bythe-mass and dimensions of the vibrator and'theelastic pivot compliance;The said resonance oscillations comprise a transverse motion of the"'effective pivot and hence, by cyclically-alterin the relative transversedisplacements of the stylus and the pivot, produce cyclic motion of the'inductor portion transverse to the magnetic; field. The effect of thesemotions on the electrical out-. put is exhibited by thefrequency-dependent terms'inthe response Equation 1. I In the .previousLetters Patent referred to above,'th'ese spurious resonanceoscillationsare controlled and damped by the provision of damping material,

shown in Figs. 1 and 2 herewith at 19, Hand 23, positioned so as to havethe maximum damping effect on the undesired spurious oscillations and aminimum effect on the desired vibrations caused by the modulation of therecord groove.- Some prior-art devices have sought to avoid thedifliculties introduced by these oscillations by using such'rigid and,therefore, massive construction that the oscillation frequencies areabove the useful range; but it is an object of this: 'disclo-' sure toshow how, even with extremely light constructions of the vibratorysystem and even though the resonant oscillation frequencies fall withinthe useful range of reproduction the effect of these oscillations may beavoidedso that there is no necessity of providing any damping.

material to control them. For if the mass distribution of the vibratorysystem is chosen so that the elastic pivot is a center of oscillationconjugate to the'stylus apex as a driving point,

no force reactions occur at the pivot to excite the resonanceoscillations and the corresponding fre By equating the above definingexpressions for we and we, we may solve for the momentof inertia of thecorresponding uniform-response vibrator, as follows:

The transverse pivot compliance C is usually very much smaller than thetorisional compliance Cs multiplied by h. This expression therefore maybe writtenin the following approximate form: Y

It follows that the product. gh is equivalent to the square of theradius of gyration of the vibratory element about the stylus apex orpoint of application of the impressed force, and this is, therefore, anecessary condition for the de sign of a uniform response vibratorysystem.

I tz- Since the geometric mean of two quantities is defined as thesquare root of their product, this necessary condition may also bedescribed. by stating'that the radius of gyration of the vibratoryelement, about the stylus apex, must be velocity having any desiredfrequency and impressed at some particular point, then this point andthe equivalent elastic pivot should form con; jugate centers ofoscillation. 'The'term ,conjugate centers of oscillation is wellunderstood in text books on "Mechanics and refers to the phenomenonwhereby an impulsive force applied at one of the centers,andperpendicularto the linejoining the two centers of oscillationproduces about the conjugatecenter a purely rotational motion. (See, forexample, Introduction to Theoretical Physics, by L. Page, pp. 115-116,published by Van Nostrand, 1938, or Handbook of EngineeringFundamentalsf'by O.-W. Esh

' bach, pp. 4-41 to 4-42, published by. Wiley,'l936.)

These terms will be Iunderstoodto describe the motion of the vibratorysystem insofar as it is controlled alone by. the effects of inertia andit may. further be understood that only these inertial effects aresignificant in determiningthe motion for impressed velocities at veryhigh frequencies. Forthis reason, the conjugatecenter of oscillation maybe referred to as an inertial axis or'an axis of percussion, asdistinguished in the above description from the axis of vibration forexcitation at low frequencieswhich is determined by the elasticity ofthe supportin means.

The preceding analysis may therefore be summarized by stating therequirement that the elastic axis must coincide with the, inertial axisfor uniform response at all exciting frequencies.

In the preferred embodiment ofthe invention shown in Figs. 12 to lathevibratory element difiersfromthat of Figs: to 11 by a tapering of thebackfiats 66, as before stated, from a narrow portion-at the junctionline to a wider portion at the junction'line 43. This tapering 'providesa reduction in the transverse-pivotcompliance Cp, which serves toincrease the values of the critical frequencies m2 and zoo, and therebyrenders the reproducer uniformly responsive over a wider range ofimpressed frequencies. In this modification, the leads 58 and 60 fromthe reproducer head are continued over the lower face 93 of thesupporting block H and form the mounting elements corresponding to theblocks of Figs. 5 to 11, to which the mounting tabs 40 are attached. Inthese figures, a magnetic circuit .is also illustrated comprising thehorse-shoe N0 of hard, permanent magnetic material, seated uponsoft-iron pole shoes I I2. The pole-shoes I I2 and the centralpole'piece H4, which may be of softiron or preferably of a hard magneticmaterial, are attached to a brass block H5, which serves toatta'ch themagnetic structure to the insulating block H by the screwsll8, and to astylus, inductor means, means whereby the stylus is connected to theinductor means, and supporting means for the inductor means comprisingmetallic ribbons disposed at an angle to each other and the planes ofwhich intersect substantially along a line that constitutes alongitudinal axis of vibration of the inductor means.

5. Vibration-translation apparatus comprising an inductor loop ofinterrupted cylindrical shell form, a cone-shaped stylus support alongthe base of which the-interrupted-cylindrical loop is secured to providerigidity, and supporting arms, one at each end of theinterrupted-cylindrical loop.

6. Vibration-translation apparatus comprising an inductor loop ofinterrupted cylindrical-shell form, a cone-shaped stylus support alongthe base'of which the interrupted-cylindrical loop is secured to providerigidity, and supporting arms, one at each'end of the interruptedcylindrical loop, the arms being disposed substantially in holdthemagnetic circuit in appropriate relation to. the inductor portion ofthe vibratory system.

Further modifications and changes will occur to persons skilled in theart, and all such are considered to fall within the spirit and scope ofthe invention, as defined in the appended claims.

What is claimed is:

1. Vibration-translating apparatus comprising a vibratory element havingconjugate centers of oscillation, means for applying vibratory impulsesat one of the centers, movable inductor-and-elastic supporting means forderiving inductively an i electric signal from the vibration of thevibratory element and for supporting the element to vibrate aboutsubstantially the conjugate center in response to vibratory impulseapplied at the said one center, and means for establishing a magneticfield in which the movable inductor means moves.

2., Vibration-translating apparatus comprising a vibratory element,movable inductor-and-elastic-supporting means for deriving inductivelyan electric signal from the vibration of the vibratory element and forproviding an effective elastic pivot for low-frequency vibrations and adistribution of mass for the element such that the radius of gyration ofthe element about the point of application of the exciting force issubstantially the geometric mean of the distance from the said point ofapplication to the center of gravity of the element and the distancefrom th said point of application to the axis through the said efiectivepivot, and means for establishing a magnetic field in which the movableinductor means moves.

3. Vibration-translationapparatus comprising a vibratory element havinga mass m and provided with an apex at a distance 9 from the center. ofgravity thereof, the element having elasticsupportin'g members adaptedto provide an axis of rotation at a distance h-from the apex and havingamoment of inertia I about an axis through the apex parallel to thefirst-named axis substan tiallyequal to l mgh(l l,

where Cp is thetransverse compliance of the I said elastic-supportingmeans.

' 4. Vibration-translation apparatus comprising plane's inclined to oneanother.

'7. Vibration-translation apparatu comprising an inductor loop'ofinterrupted cylindrical-shell form, a cone-shaped stylus support alongthe base of which the interrupted-cylindrical loop is secured to providerigidity, and supporting arms, one at each end of the interruptedcylindrical loop, the arms being disposed substantially in planesinclined to each other, and the line of intersection of the planesconstituting a longitudinal axis of vibration of the inductor loop.

8. Vibration-translation apparatus comprising an inductor loop ofinterrupted cylindrical-shell form, a cone-shaped stylus support alongthe base of which the interrupted-cylindrical loop is secured to providerigidity, and supporting arms. one at each end of the interruptedcylindrical loop, the supporting arms comprising sections of warpedsurfaces providing for a transverse compliance of the stylus apex one totwo time larger than the vertical compliance of the stylus apex.

9E Vibration-translation apparatus comprising a stylus, inductor means,means whereby the stylus is connected to the inductor means, andsupporting ribbons for the inductor means com prising sections of warpedsurfaces providing for a ratio of transverse to vertical compliance ofbetween one and two.

10. Vibration-translation apparatus comprising a stylus, inductor means,means whereby the stylu is connected to the inductor means, and

supporting ribbons for the inductor means comprising sections of warpedsurfaces providing for a ratio of transverse to vertical compliance ofbetween one and two, the supporting means comprising arms disposedsubstantially in planes inclined to each other. 11.Vibration-translation apparatus comprising a stylus, inductor means,means whereby the stylus is connected to the inductor means, andsupporting means for the inductor means comprisin sections of warpedsurfaces providing for a ratio of transverse to vertical compliance ofbetween one and two, the supporting means comprising arms disposedsubstantially in planes inclined to each other, and the line ofintersection of the planes constituting a longitudinal axis of vibrationof theinductor means.

v12. Vibration-translation apparatus comprising a stylus, a support forthe stylus and a combined mechanical-electrical connector-jack forsupportis larger than the compliance for longitudinal sulated from themating portions, and means for simultaneously clamping together themating electrical leads and the mating portions.v

l3. Phonographic apparatus comprising 1 a stylus adapted to be disposedin contact with a record groove, combined inductor-and-elasticsupportingmeans, the said supporting means comprising-two ribbon members providingat the stylus simultaneously a large compliance for transverse-motion, acompliance for vertical motion lower than thefirst-named compliancefanda compliance for longitudinal motion lower than the first two namedcompliances.

14.,Phonographic apparatus comprising; a stylus adapted to be disposedincontact with a record groove, combined inductor-and-elasticsupportingmeans, the said supporting means comprising two ribbon membersproviding-at the stylus simultaneously a large compliance for transversemotion, a compliance for vertical motion lower than the first-namedcompliance, and a compliance for longitudinal motion lower than thefirst two named compliances, and having the vibrational axis determinedby the inertia of the parts coincident with the axis determined by theelastic supporting means.

15. Vibration-translating apparatus comprising a stylus, a stylussupport comprising a conical shell having an integral cylindricalsection atits base, movable inductor-and-elastic-supporting meansattached to the cylindrical section constituting the mechanical systemto which the stylus delivers energy and the electrical system forconveying' the energy, and means for establishing a magnetic fieldtransverse to the inductor-andlastic-supporting means, theinductor-and-elastic-supporting means being unitary to constitute thesole support for the conical shell and being rigid to move in themagnetic field as a single whole without relative vibration of theelectromagnetically active portions of the inductor means.- e

l6. Vibration-translating apparatus comprising a stylus, a stylussupport comprising aconical shell having anintegral cylindrical sectionat its base,inductor means attached to the cylindrical section,-.;means,for establishing a magnetic. field transverse to the inductor means,and elastic supporting means comprising two metallic ribbons,

attached to the cylindrical section at junction lines forming equalangles with a;plane perpendicular to the axis of the cylinder andattached at their'other ends to supporting means on junction linesforming equal, but smaller-than the firstnamed, angles with thesaid-plane. i

' 17. Vibration-translating apparatus comprising a stylus, a stylussupport comprisinga conical shell having an integral cylindrical sectionat its base, inductor means attached to the cylindrical section, meansfor establishinga magnetic field transverse to the inductor means, andelastic-v supporting means comprising two metallic ribbons attached tothe cylindrical-section at junction-lines forming angles of, about 25with a plane perpendicular to the axis of the'cylinder and attached attheir other ends to supporting of about motion.

19. Phonograph apparatus comprising a stylus adapted to'be disposed incontact with the sound track of a record, a support for supporting thestylus in contact with the sound track, the support havinga rear portionfixed at one end and a movable portion carried at the other end, andmeans for mounting the stylus at the free end of the movable portiononlyto, constitute the movable portion the sole support for the stylus,the

vibratory axis established by the support being disposed substantiallyat the center of oscillation conjugate to the point of contact betweenthe stylus and the sound track.

20. Phonograph apparatus comprising a stylus adapted to be disposed incontact with'the sound track of a record, asupport for supporting thestylus in contact with the sound track, the support having a rearportion fixed atone end and a movableportion carried atthe other end,and means for mounting the stylus at the free end oi the movable portiononly to constitute the movable portion the sole support for the stylus,the

movable portion being shaped to prevent substantial mechanicaldeformation thereof in response 1 to movement of the stylus along thesound track, and the rear portion being flexible in order that it mayflex mechanically in response to movement of the stylus along the soundtrack, thereby to cause the movable portion to move substantially as awholein response to-movement of the stylus along the sound track, thevibratory axis established by the upport being disposed substantially atthe center of oscillation conjugate to the point of'contact between thestyl ,s and the sound track.

21 Phonograph apparatus comprising a stylus adapted to be disposed incontact with the sound track of a record, means for creating a magneticfield, a support for supporting the stylus incontact with the soundtrack, the support having a rear portionfixed at one end and a movableinductor portion carried at the other end, and

means for supporting the stylus at the free end of the inductorportiononly to constitute, the inductor portion the sole support for thestylus, the, inductor portion being disposed in the magnetic field inorder that electric currents may flow therein in response to movement ofthe stylus along the sound track, the inductor portion being shaped toprevent substantial mechanical deformation thereof in responseto-movement of the stylus along the sound track,v and the rear portionbeing flexible in order that it may flex mechanically in response tomovement of the stylus along the sound track, thereby to cause theinductor portion to move substantially as a whole in response tomovement of the stylus along the sound track, the vibratory axisestablished by the support being disposed substantially at the centerof' oscillation conjugate'to the point of contact between the stylus andthe sound track.

22. Phonograph-reproducing' apparatus comprising a stylus, vibratorysupporting means for the stylus, and electrical means for deriving anelectrical voltage from the vibrations of the supporting means, thesupporting means being disposed to provide great flexibility for lateralmotion of thestylus, whereby the recorded sounds may be faithfully;reproduced in the electrical output, and the supporting means beingdisposed so as to provide, great flexibility for vertical motion of thestylus without producing a corresponding electrical output, therebyremoving the prin.

